How to fix Request.Url.Scheme returning http instead of https on load balanced site - Blog post: https://aaronsadler.dev/2021/march/19/how-to-fix-request-url-scheme-returning-http-instead-of-https-on-load-balanced-site/

HttpOverrides.cs

public sealed class HttpOverrides : IHttpModule
{
    void IHttpModule.Init(HttpApplication app)
    {
        app.BeginRequest += OnBeginRequest;
    }

    private void OnBeginRequest(object sender, EventArgs e)
    {
        HttpApplication app = (HttpApplication)sender;

        string forwardedFor = app.Context.Request.Headers["X-Forwarded-For"]?.Split(new char[] { ',' }).FirstOrDefault();
        if (forwardedFor != null)
        {
            app.Context.Request.ServerVariables["REMOTE_ADDR"] = forwardedFor;
            app.Context.Request.ServerVariables["REMOTE_HOST"] = forwardedFor;
        }

        string forwardedProto = app.Context.Request.Headers["X-Forwarded-Proto"];
        if (forwardedProto == "https")
        {
            app.Context.Request.ServerVariables["HTTPS"] = "on";
            app.Context.Request.ServerVariables["SERVER_PORT"] = "443";
            app.Context.Request.ServerVariables["SERVER_PORT_SECURE"] = "1";
        }
    }

    void IHttpModule.Dispose()
    {
    }
}

Web.Config

<system.webServer>
    <modules runAllManagedModulesForAllRequests="true">
        <add name="HttpOverrides" type="Namespace.HttpOverrides" preCondition="integratedMode" />
    </modules>
</system.webServer>